It has been demonstrated that the immunological potential declines in old age. We have found that an increase in suppressor T cell activity is the first age-related immunologic lesion in mice. This is followed by a decrease in helper T cell function and finally by loss of B cell function. Spleens from mice treated with cyclophosphamide were found to contain cells capable of suppressing the secondary response of syngeneic spleen cells. These suppressor cells were not T cells and were present in the spleen of unimmunized, cyclophosphamide-treated mice. Conceivably these suppressor cells may be involved in the elicitation of autoimmune phenomena in aged mice.